The invention relates to a mixer head for reactive mixing of two or more plastics material components in a mixing chamber, which are ducted simultaneously and conjointly into the mixing chamber during the infeed stage, under pressure and via a dosing valve in each case, and pumped back in a circulatory flow which flows through the dosing valve during the recirculation stage.
A mixer head of this nature serves the purpose of processing reactive plastics material components, one plastics material component often being a polyol for example, and the other plastics material component an isocyanate (as in accelerator). An essential field of application of a mixer head of this kind is the production of seals or joints closed upon themsleves, for example such as utilised in the area of bulbs connecting the bulb cap and the bulb glass. Another purpose of application is the production of encircling door joints in the case of instrument cabinets and the like.
Small quantities only of the plastics material are utilised for the production of peripheral joints of this nature. In a typical case of application, the mixing ratio between the one component and the other amounts to 5:1 and an expulsion speed of 1 gramme per second is utilised. It is known however that the processing of the plastics materials in such small volume encounters great difficulties in the production of a peripheral joint by means of a mixer head as described above, the mixer head is displaced along processing track, so that during the infeed stage, a plastics material extrusion of a diameter of 5 mms for example is deposited in the groove of a bulb cap at a speed of 20 cms per second, for example. The mixer head travels along the groove and deposits its extrusion in the groove. Since the groove is encirclingly closed upon itself, the mixer head returns to its starting point and has to establish a seamless connection between the two extremities of the plastics material extrusion. The connection is established by the fact that the plastics material strings flow together before being foamed reactively, so that an expanded joint having a smooth outer skin and an uniform height is the result after a completed manufacturing operation.
This aim was secured but inadequately until now, because the mixer heads known until now deposited a greater quantity of the plastics material extrusion per unit of time at the beginning of the delivery stage than at the end. This was because a pressure peak occured upon passing from the recirculatory operation to the delivery stage, which had the result that a greater quantity of plastics material was deposited per unit of time at the beginning of the delivery stage than upon termination of the delivery stage. The pressure peak was caused primarily because the pressure of the flow of fluid was not controlled and the supply and return lines acted as pressure accumulators in view of their flexibility, which were charged at the time of recirculation and which discharged at the start of the delivery stage, so that a greater quantity of plastics material was ejected at the beginning of the delivery stage.
Because of the air charge in the fluid, the latter itself is also compressible, which equally contributed to the problem that a greater quantity of plastics material was deposited at the beginning of the delivery stage, than later.
The invention consequently has as its object to develop a mixer head of the kind defined in the foregoing, in such manner that the fluid pressure is kept constant with precision during the delivery and recirculation stages, so that the generation of the pressure peaks referred to in the foregoing no longer occurs.
To resolve the problem posed, the invention is characterised in that the dosing valve is constructed as a pressure control valve which has post-connected to it in series in each case a recirculation valve equally constructed as a pressure regulator valve, in such manner that either the dosing valve or the recirculation valve is switched to the throughflow setting.
It is thus a feature of the invention that the dosing valve and the recirculation valve are in each case constructed as pressure control valves and that both valves are connected one behind the other in series. It is accomplished thanks to the construction of the dosing valve as a pressure control valve, that the pressure within the fluid is kept constant rigorously, irrespective of whether the delivery stage or the recirculation stage is in progress.
By connecting a dosing valve in series with a recirculation valve in such a manner that only the dosing valve or the recirculation valve is switched to the throughflow state, it is accomplished that a pressure peak does not occur upon passing from the recirculation stage to the delivery stage.
In this connection, it is preferable that the control pressure of the dosing valve should be equal to the control pressure of the recirculation valve, so that no pressure peak is engendered upon switching from the recirculation valve to the dosing valve, because the conveying system is not alerted.
Thanks to the series connection of two analogous valves having identical control pressures, the conveyor system is consequently always preset so that only the correct valve traversed by flow during the recirculation stage, the dosing valve allocated to the mixing chamber is traversed by flow only such manner that the fluid is ducted past the nozzle bore closed off by a needle, to prevent settling of the fluid. During the delivery stage, a balance prevails between the hydrostatic force of the flow of fluid and the adjustable force of a spring. The spring is adjusted however that the needle clears the nozzle bore. If the fluid pressure acting at the one side of the needle increases, the increased return force of the spring then acts at the other side of the needle, so that the needle always remains balanced and opens the nozzle bore in the form of an annular gap of ever constant cross-section.
During the delivery stage, the recirculation valve is closed and the fluid flow impelled by the pump is led into the mixing chamber of the mixer head via the specified pressure controlled dosing valve.
The fluid flow of a single plastics material component only is described in the following. The fluid flow of the other component is wholly identical, so that the description of the one fluid flow is sufficient.
The pressure control of the dosing valve and of the recirculation valve is performed according to the restrictor principle. What is described is a constantly acting throttle valve of adjustable restrictor cross-section, because the tip of the needle clears an annular gap in the open state of the dosing valve whose size depends on the pressure within the system and on the preloading force of the spring.
All pressure control valves (flow valves) are thus covered by the principle of the present invention, irrespective of which throttling principle they utilise.
To secure reliable closing and opening of the throttle and recirculation valves, it is preferred that the valves should be preset by means of air control. The control air is applied in such manner that the dosing valve is kept closed securely by means of the control air during the recirculating operation, and that the recirculation valve is not acted upon by air and perform its regulatory function.
Conversely, the dosing valve is not acted upon by air during the delivery operation and performs its regulatory function, whereas the recirculation valve is reliably closed by means of the control air.
Extremely small quantities of plastics material can be processed successfully and uniformly per unit of time with the pressure control system described. In the case of one component, a discharge of no more than 0.2 grams per second is specified, in which connection this discharge quantity is kept precisely constant over a protracted period with success, using the mixer head of the present invention.
The invention is described in particular in the following with reference to drawings illustrating one mode of embodiment only. In this connection, other features and advantages of the invention emerge from the drawings and their description.